This invention relates to a recording and reproducing apparatus for a magnetic disk which is used for an electronic computer, a word processor, and the like, and more particularly to a stepping motor controlling circuit for driving and controlling a stepping motor.
In a recording and reproducing apparatus for a magnetic disk, a stepping motor is rotated to step and position a writing/reading magnetic head to a desired track in order to write information onto the desired track of a magnetic disk or to read information from the desired track. Commonly, a track located at an outermost circumference of a magnetic disk is called zero track and is regarded as a reference track for positioning a head to a desired track.
Typical ones of conventional stepping motor controlling circuits and zero track detecting circuits are illustrated in FIG. 3. Reference numeral 1 denotes a stepping motor of a 2-phase excitation bipolar driving method, and a head (not shown) is moved by one track distance by switching the exciting mode of the stepping motor 1 twice. Reference numeral 2 denotes a driver circuit for driving the stepping motor 1, and 3 an excitation controlling circuit responsive to a stepping signal c from a computer (not shown) to drive the stepping motor 1 and also to a feeding direction signal g for switching the exciting mode of the stepping motor 1. Reference numeral 5 designates a switch circuit for switching a power source to the stepping motor 1 on and off. The switch circuit 5 is triggered by a falling edge of a power on signal a which indicates whether a power source is switched on to the apparatus or not or of a stepping signal d to supply a voltage of 12 V to the driver circuit 2 and also to the stepping motor 1 via the driver circuit 2, but even if a power on signal a is active, when no stepping pulse c is inputted, the switch circuit 5 stops supply of the voltage of 12 V to the stepping motor 1 and the driver circuit 2. The stepping motor 1 has a pair of coils .phi.A and .phi.B, and the exciting mode thereof is determined depending upon a combination of directions of electric currents flowing through the coils .phi.A and B. Four such combinations are indicated below:
1. AB phase mode . . . an electric current flows in a direction of an arrow mark x through the coil .phi.A while an electric current flows in a direction of an arrow mark y through .phi.B; PA0 2. AB phase mode . . . an electric current flows in a direction opposite to the direction of the arrow mark through the coil .phi.A while an electric current flows in the direction of the arrow mark y through the coil .phi.B; PA0 3. AB phase mode . . . an electric current flows in the direction opposite to the direction of the arrow mark through the coil .phi.A while an electric current flows in a direction opposite to the direction of the arrow mark y through the coil .phi.B; and PA0 4. AB phase mode . . . an electric current flows in the direction of the arrow mark through the coil .phi.A while an electric current flows in the direction opposite to the direction of the arrow mark y through the coil .phi.B.
A head is stepped between adjacent tracks by sequentially switching the electromagnetic mode twice in response to each stepping pulse c. It is to be noted that the exciting mode at an even numbered track is determined as the AB phase while the exciting mode at an odd numbered track is determined as the AB phase.
Meanwhile, reference numeral 7 denotes a zero track sensor for detecting whether the head is positioned on the zero track or not. The zero track sensor 7 includes a light emitting diode D1 and a phototransistor Q1 which are disposed in opposing relationship to each other. An electric current always flows across the anode and the cathode of the light emitting diode D1 to emit light therefrom. The phototransistor Q1 remains on while it receives light from the light emitting diode D1, an electric current flowing across the collector and the emitter of the phototransistor Q1. Reference numeral 8 denotes an inverter which receives an input of a sensor output voltage i, and 9 a NOR gate which receives inputs of an AB phase mode signal h from the excitation controlling circuit 3 and of an sensor output signal j from the inverter 8.
Now, operations of the circuit shown in FIG. 3 will be described with reference to a timing chart of FIG. 4 by way of an example where the head moves from the third track to the zero track. Before a point of time t4, the head is positioned on the third track, and the motor exciting voltage d is not supplied. When a stepping pulse c is produced at the time t4, the motor exciting voltage d is supplied as triggered by a rising edge of the stepping pulse c so that the exciting mode of the stepping motor 1 is changed over from the AB phase mode at the third track to the AB phase mode at the second track through the AB phase mode. While the exciting mode remains in the AB phase mode, the AB phase mode signal h from the excitation controlling circuit 3 becomes an active low level from a high level. Then, when a second stepping pulse c appears at a time t5, the exciting mode is changed over to the AB phase mode at the first track through the AB phase. In the meantime, since part of a carriage (not shown) on which the head is mounted comes between the light emitting diode D1 and the phototransistor Q1 to begin to intercept light to be received by the phototransistor Q1 from the light emitting diode D1, the output voltage i of the zero track sensor 7 gradually rises, and when the output voltage i exceeds a predetermined level (2 V), the sensor output signal j changes from a high level to a low level via the inverter 8. In other words, the sensor output signal j is switched to an active low level at a stage when the head comes to the first track. Finally, when a third stepping pulse c appears at a time t6, the exciting mode is changed over to the AB phase mode at the zero track through the AB phase mode. When the exciting mode thus becomes the AB phase mode, the AB phase mode signal h again becomes an active low level. Thereupon, since the AB phase mode signal h and the sensor output signal j both become an active low level, the zero track signal k which is an output of the NOR gate 9 becomes an active high level. Thus, the zero track signal k of the high level which indicates that the head is currently positioned on the zero track is transmitted to the computer.
As described above, in the conventional arrangements, the exciting modes at the zero and first tracks are the AB phase and the AB phase, respectively, and are thus different from each other, but the output signals j of the zero track sensor 7 at both tracks are both active. Meanwhile, a control IC (a gate array) not shown has a characteristic that when a power source which was once turned off is again turned on, the control IC initializes the exciting mode of the stepping motor 1 to the AB phase mode. However, if the power is turned on while the head remains positioned on the first track, since the exciting modes at the zero track and the second track are both the AB phase mode, the stepping motor 1 cannot be stepped from the AB phase mode to the AB phase mode in either direction. Accordingly, it is a problem that since the AB phase mode signal h and the output signal j of the zero track sensor 7 are both at an active low level although the head remains actually positioned on the first track, the zero track signal k becomes an active high level which indicates that the head is positioned on the zero track.